[QUESTION/ANSWER] Have you tried replacing 1A2 phone's incandescent lamps with LEDs?

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Greg Ercolano

Nov 26, 2019, 11:49:16 PM11/26/19
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> Have you ever tried replacing the 51a lamps in the 1A2 phones with LEDs?

Yes, I've done tests, just to check electrical characteristics and lighting, but not committed a whole phone to them.

Here's a side-by-side test of a "white" LED with a series 1K resistor vs. the 51a incandescent, both running at 12VDC from the Seriss 1A2 KSU:

The 10mm LED I used fits perfectly into this 256x key's lamp shroud, and gives an icy-white brilliant result (left, Line 1), as compared to the more warm orange look of the 51a incandescent (right, Line 2).

There are many benefits with LEDs; brighter light, longer life, less heat, and operate on less current.

The 51a lamps typically draw about 40mA each, which adds up when you have a lot of phone extensions and all lines in use. For instance, 8 phones with all 5 lines in use, the lamps alone would draw 1600 mA (1.6 amps).

Depending on the current limiting resistor you use on LEDs, they'll consume about 1/2 the incandescents, while giving more brightness, less heat, and longer life span.

The LED I used above is a "cool white" 10mm diameter LED (around $0.30 each), the 1K resistor is off-screen.
I also tried some 5mm diameter white LEDs (around $0.18 each) with similar brightness and power results.

Since the Seriss 1a2 KSU uses 12v DC for lighting the phone lamps, the LEDs don't have any kind of flicker while lit. (With an AC source they'd flicker at a 60Hz rate, as they'd only be lit on one-half of the AC cycle, and off the other half, due to the inherent diode characteristic of LEDs).

You can probably purchase "pre-wired" LEDs with the resistor wired up (such as these), and just attach the wire leads directly to the lamp screws (e.g. "L1" and "LG") on the phone's terminal board, by-passing the lamp sockets. You may want to increase the resistor values if the LEDs are too bright for your tastes.

I'm not currently aware of LEDs designed to fit into the socket of a 51a, but there may easily be some I'm not aware of. LEDs are quickly replacing incandescent lamps in all industries, so if there aren't any now, I wouldn't be surprised if they become available in the near future.

FWIW I've not had trouble finding replacement 51a bulbs; they're still a common format, and are pretty cheap to get a box of them online, either on Ebay or Amazon. For example, Sylvania makes some that come in a box like this for around $5 for a 10 pack:

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